Supernovae Search
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Tim
Puckett's 60 cm f/8 R-C scope that is used to monitor
supernovae. |
Estimating
the supernovae magnitude (I)
Each
year between one and four supernovae (SN) are discovered in Virgo cluster
by amateurs. Early observations of amateur SN magnitude observations suffered
from large zero-point corrections (in some cases of up to 0.8mv, see, when
compared to mean photometric data that appeared in various papers written
at the time). This was possibly due to an inadequate source of published
data pertaining to the subject matter ie. sequenced magnitudes from which
to compare.
However,
this is not the only criteria which factors into making a viable
estimate. I will attempt to list some of these variables, both
systematic and random which the observer should consider...
Personal equation (Systematic)
This
includes : the telescope (aperture, quality of optics, position angle
equation[see below]), seeing conditions of the site used to make the
observation (rural or urban; dark or light polluted), weather
conditions (humid or cold), the ability of the observer (number of
years observing, color determination, sensitivity, etc.), the age of
the observer young or older and (this is not included in too many
studies of personal equation), the attitude of the observer at the
time of the observation (stressed, relaxed, hard day at the work
place, mother-in-law in town for her annual visit, mother-in-law
leaving town from her annual visit , etc.)
To
read : Estimate
your personal equation
Charts (random)
Currently there are
many sources which can assist the observer in not only making
magnitude estimates of a newly found supernova, but can also help
determine if a suspect star is a possible supernova candidate. Other
comparison charts can be obtained from the AAVSO,
UK Supernova Patrol, and the M-1 Supernova Network in Madrid (Also
see the next link for additional resources). If you have access to a
university library, articles written over the years concerning past
SNe events will also give very detailed information on photometry,
light curves, and the mechanics involved concerning the event and
will usually include many comparison stars.
Useful
links on Supernovae
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SN
1993J in M81 by T.Lombry
on May 14, 1993. It was the brightest (Mv +9.91 in U) after
SN 1987A. |
SN
1995BW (yellow) and SN 1997W (blue) in galaxy NGC664 recorded on feb 1, 1997 by
Carl
Hergenrother. |
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Other methods
Several
other methods exist for obtaining reference material relating to
starfields. This process has been used for years by the master visual supernova
discoverer...The Rev. Robert O. Evans of Coonabarabran, Australia.
In fact his inspiration has been passed on to Dana Patchick of
California (discoverer, 1987L) who accomplished a similiar
feat....this utility is the photographing of the Palomar Sky Survey
(in Rev. Evans case additional surveys were also made available to
him) into a slide format!
The
idea is to have access to an astronomical library that has a
collection of the POSS (Palomar Sky Survey) plates, then construct a
macro lens attached to a 35mm-camera to accomplish close-up views
and snapshots of individual galaxies. In proceeding in this manner
you cut down on the costs of actually having to purchase the charts
(available from Cal-Tech for $36 for six-6x6 degree copies of the
POSS plates. Note that this price may have gone up, but that was the
price a few years ago.
Ed.
Note: The acquistion of these plate copies for the entire Virgo area
can be obtained in 4 sets (of six), and the work of enlarging to
snapshot-size charts may then be accomplished at ones leisure, this
then breaks down to pennies per galaxy. Rev. Evans has thousands of
such slides, Dana has spent a year getting several hundred to a
thousand images for use at the telescope and for reference. The main
idea for this project was to have a ready reference available in the
event some star was spotted that appeared "out of place"
near or involved in some galaxy.
Like
any experienced observer these gentlemen have enough skill to be
able to visually estimate [within a few tenths of a magnitude] a
particular star. (that is why the Experienced observer would stand a
better chance at this project than one who is not familiar with
making magnitude estimates). While this regimen has some pitfalls it
will give the observer some satisfaction that he is able to
"weed" out any stars that might already exist near some
galaxy, and that he will not have to call someone long distance on
the telephone, or energize a verification team to verify some
spurrious object....in essence you then become somewhat
self-sufficient in the verification department.
Making estimates... a suggestion
There
are many unavoidable criteria that must be considered in making a
valid magnitude estimate, some will be suggested here, however the
professional community will be better apt in determining some
exacting standards of these values.
One
must consider that a SN suspect will, to the visual observer (or CCD
imager) appear simply as an additional point of light in a
particular galactic starfield. But to the professional astronomer
Galactic Absorption or the amount of reddening along our line of
sight to the host galaxy must be obtained, as well as Internal
Absorption, or the amount of absorption contained within the various
galaxies where these events takes place.
Other
values also include where in the galaxy the event occurs. Absorption
values might, for instance be higher if the event takes place within
an HII region, etc. (SN 1994I in M51 [NGC 5194])
displayed a more subluminous posture, possibly due to this fact
absorption was ~1.8 higher along our line of sight to the event).
(Ed.
Note: SN 1994I was designated a type Ic SN, which as noted in,
followed a slower decay than the mean type Ia, could absorption have
been the culprit, or where different explosive scenarios at work
here?).(Additionally, one should keep in mind that when making
magnitude comparisons using CCD imaging to adhere to the following
suggestion from B.Skiff [Lowell Observatory] : "it should be
noted once again that in principle there is no way to transform data
for emission-line objects like novae and supernovae to any standard
photometric scale based on ordinary stars. The spectra are simply too dissimilar to avoid systematic
errors between different filter/detector combinations.
The
problem is most acute toward the blue, but workable with broadband
filters in the red - as long as each system is well
calibrated..." (message to Novanet, 3/7/95). So what criteria
should the visual/CCD observer take into consideration? Firstly,
(although perhaps not of too much consequence). Atmospheric
Extinction can be factored in determining the displacement in
degrees from the zenith...using the formula :
cosZ= (sin a)(sin b) + (cos a)(cos b)(cos H)
with:
Z
= distance from the zenith to geometric horizon nearest object
a
= observer's latitude on earth
b
= declination of object being measured
H
= hour angle of the object being measured.
Once the correct zenith distance has been determined,
the amount of atmospheric extinction can be determined by the formula :
M=0.35
(sec z1 - sec z2)
with:
M
= extinction coefficient
z1
= zenith angle of highest star
z2
= zenith distance of star closest to horizon.
To
read : Tim Puckett’s Award-Winning Ambition,
Sky & Telescope, 2012
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SN
1998BU in NGC3368 recorded on May 27, 1998
from CTIO. |
SN
2000CJ (blue) in galaxy NGC6753 recorded by Nick
Suntzeff. Documents ISN. |
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Position angle equation
Angle error in making visual estimations arise from the physiology of
human vision. Color error can be minimized by selecting comparison
stars similiar to the variable color index. Angle error results
change in the apparent difference between a variable and a
comparison star when their orientation changes relative to the line
between the observers eyes.
Observations
made using altazimuth instruments with fixed eyepiece positions are
particularly prone to angle error - as are bonocular and naked-eye
observations because the observer normally faces in the direction of
the variable and uses his/her body as an altazimuth mounting, the
eyes remaining parallel to the horizon.
The
observer will face east to view a star field rising in the east,
then face west to observe the same starfield setting. As a result,
the star field will appear completely reversed between the two
observations, east changing from down to up, north from left to
right.
To
confirm this suspicion, I performed an observational experiment.
Reclining horizontally, I made one series of estimates with my feet
toward the east and a concurrent series of estimates with my feet
toward the west. I tilted my head back beyond the zenith when
necassary".
Ed.
Note: My Williams' estimates indicated a displacement or magnitude
differance from the above methods of ~0.25m while observing the
variable star BX AND. He further states: "....These
observations were made on the same night, by
the same observer, with the same telescope and comparison star
values.....This should convince visual observers of the need to
maintain the same orientation of the star field for all estimates of
a variable star..."
Aperture-dependance
The
Argelander method is the most popular for estimating magnitudes. By
assigning a step to each comparison star in a pair that brackets the
brightness of the star in question, the magnitude of the latter can
be inferred. The step is chosen from a universal scale of steps.
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M106
aka NGC 4258 located 25 million light-years away (19'x8', Mv +8.4). Two supernovae were observed in this Seyfert
galaxy (SN 1981K and SN 2014bc which appears above the nucleus in this picture taken
by Lásló
Szeri. Here is M106 without the supernova.
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An
obvious extension of the Argelander method, is the use of additional
pairs of comparison stars. More observers now use this method
because it leads to an improvement in the accuracy by reducing
random errors, with very little increase in effort and observing
time. Not only are random errors reduced with this practice, but
sesitivity problems (comparison stars being red) are somewhat
diminished, as well as problems introduced by erroneus magnitudes.
The Argelander scale of steps is, in principle, universal,
but each observer applies it in a different way. Usage of comparison
stars with wrong magnitudes contributes in an important way to
obtaining false light curves.
On
another note some observers will find inconsistancies between
magnitudes and their observations. The problem here is that
reference charts used may have been drawn up decades ago. When these
inconsistancies become apparent, and lacking any recent
photoelectric measurement updates, estimates of the magnitudes
should be attempted visually (this is where experienced observers
stand a better chance). This may be accomplished from any two or
more comparison stars whose magnitudes can be determined.
When
measurements from various observing sessions are averaged and the
new magnitudes implemented, better light curves can be obtained.
For an observer sensitive to red, the use of a red comparison
star makes the variable appear fainter than it really is, therefore
it seems convenient to use a magnitude for that comparison star that
agrees with what the observer sees. From this point of view each
observer is a particular "photometer" and the question
arises as to wheather it makes sense, strickly speaking, to collect
data from slightly different "photometers" Finally, there are
the effects related to different instruments and observing conditions. These effects are less
important. Some time ago (the authors), we observed a possible
aperture-dependance. We undertook a study of different apertures.
The
observing method included the techniques outlined above and also the
out-of-focus technique, which is useful in estimating stars that are
close in brightness or red.. To avoid position-angle problems as
much as possible, it is also good observational practice to observe
from one side of the telescope and then the other - if using a
reflector - to compensate for the uneven response of the retina. We
assumed a linear relation between observed magnitude and aperture
(borrowed from an idea by Bobrovnikoff and Morris...[note....some
possible flaws involving associated observational methods, involving
comets, were mentioned in the colloquium proceedings: S.Edberg,
pg.95-99], however studies by Bateson {IAU Colloquium #46} did not
confirm this for variable stars...ed]. thus :
m(obs) = m(stand) + a[alpha](A -5)
where:
m(obs) = is the estimated magnitude
m(stand)
= is the standard magnitude defined to be the magnitude
corresponding to a previously chosen standard aperture (in
this case 5 cm)
A
= is the aperture of the instrument in cm.
a[alpha]
= is obtained from a least-squares fit. The convention plots
magnitude correction - that is estimated magnitude, minus standard
magnitude - verses aperture.
The
authors found a definate trend: the larger the telescope, the
fainter the star. This convention incorporated several hundred
observations made, spanning about a year. Some observations taken,
where not intended for use in this particular study. We (the
authors) ruled out red stars in order to avoid color problems,and
used the observational techniques quoted above. However, we think
that a more sensible parameter is the limiting magnitude of the
telescope, which depends on both aperture and observing conditions,
thus including several factors in a single parameter. The limiting
magnitude is a barrier near which stars are more difficult to
observe, but at the same time differances in brightness are more
easily detected.
Much
brighter stars may not be so clearly distinquished (when plotting
Argelander step methods versus distance) from limiting magnitude.
For a given differance in brightness between a pair of stars, say
0.5m, the step assigned to this separation tends to be higher as we
approach the limiting magnitude. In theory, at the limiting
magnitude, the step would be infinite if one of the stars were to
lie on the border and the other below it.
Furthermore,
a straight line with a non-zero slope is evident for values less
than four magnitudes above the limiting magnitude..." [a figure
was included in their article, however it was not reproduced here]
(ed. note: The information revealed here was shortened, and
indicates the authors EV and PV's intent....This method could be
considered and is used here as a point of conversation....the
authors might have already provided a more updated and/or current
result).
Conclusion
With
all considerations factored, the visual observer should have had
some previous magnitude estimating experience. Comparison stars
should be well sequenced, and stars with varying color indices (if
possible) should be used as the event decays (Note: Comparison stars
within the same field of view will have almost non-existent
differances in atmospheric extinction from that which might be
present)(See Light and Color Curves).
The
ideal situation to help eliviate scatter found in many event
estimations, would be a standarized comparison sequence that all SN
enthusiasts could have access to! The Guide Star Catalog is perhaps
the largest, most readily available source, but unfortunetely this
work suffers from some disparity amongst its magnitude sequences.
Hope for the future might include transformation formulae, or a
better more accessible method for standardizing magnitudes around
various galaxies, however this work would probably approach biblical
proportions.
In
summation, The visual observer should attempt to bracket magnitudes
by following V band reference star magnitudes compared to the SN (if
possible). Some attention should be given to the color index (B-V)
model given in the section on light and color curves (next page) to possibly
eliminate some of the magnitude scatter.
It
is important to remember that accurate magnitude measurements need
not equate a stress-free, non-smoking, non-coffee drinking
individual, who owns a perfect telescope, observes from the right
position from a sub-arc second photometric site at 72 degrees, who
is not color sensitive, and ranges in age from 22-35 years old, who
was weaned at birth with a telescope....it just takes a bit of
determination and practice, practice, practice.
Second part
Supernovae
Light Curves
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